Press cover, use thereof, and press roll, shoe press and use of a reinforcing thread in a press cover

ABSTRACT

A press cover contains at least one polymer layer in which a reinforcing structure is embedded. The reinforcing structure contains at least one reinforcing thread, wherein the material of the reinforcing structure and that of the at least one polymer layer are selected such that the visible light transmittances thereof differ from each other.

The invention is based on a press cover, in particular for a pressingdevice for treating a fibrous web, for example to smooth or dewater thelatter, in detail according to the independent claims. The inventionalso relates to a press roll, a shoe press and the use of a press coverin such, in detail according to the subordinate claims.

Pressing devices such as shoe presses have for a long time been aconstituent part of modern paper machines. They substantially comprise ashoe that is arranged to be stationary (also called a press shoe), whichextends in a cross-machine direction, and a press cover circulatingaround the stationary shoe. Said press cover is deformable andsubstantially assumes a tubular shape in operation. The shoe is shapedsuch that it forms a press nip with an opposing roll. The press nip isdefined by the contact surface of the opposing roll in the shoe. Theshoe is movable and can be moved onto the opposing roll.

Enormous requirements in relation to its stability are placed on thepress cover, specifically with regard to surface hardness, resistance topressure, temperature and hydrolysis. In addition, during operation thepress cover is subjected to high alternating flexural loadings. As itruns in at the shoe edge—upstream of the press nip as viewed in thedirection of rotation of the press cover—firstly flexure under acomparatively small radius takes place. This changes immediately into anopposite flexure as it passes through the press nip. As it runs out atthe other shoe edge, that is to say—downstream of the press nip asviewed in the direction of rotation of the press cover—an oppositeflexure takes place again. This deformation of the press cover as itruns in and out is also designated as an alternating nip. It can easilybe seen that the tendency of the press cover to fracture, in particularat this point, as a result of the high mechanical stress is very high.Accordingly, the prior art discloses many measures which are intended toincrease the stability of the press cover.

The press cover must thus be sufficiently flexible that it can be ledaround the shoe, it must be sufficiently rigid in order that it is nottoo highly deformed or compressed under the press load in the nip, andit must be sufficiently wear-resistant. Therefore, press covers consistof a single-layer or multi-layer polymer layer, preferably made ofpolyurethane, into which reinforcing threads in the form of laid orwoven fabrics can be embedded.

The present invention relates to such generic objects mentioned at thebeginning.

Press covers known from the prior art tend to premature failure duringnormal operation as a result of—often only local—overloading in the nip.Such overloading arises when, during what is known as a lump passage, aforeign body goes through the nip. Such overloading often leads to thereinforcing threads or the polymer layer in which they are embeddedtearing. A press cover which, for example, is oil-lubricated from theinside, can become leaky, so that the oil comes into contact with thefibrous web to be produced. As a result of this, the press cover must bechanged. In practice, this leads to unplanned stoppages of the pressingdevice and therefore to increased, costly downtimes.

It is accordingly an object of the invention to specify a press coverwhich avoids the disadvantages of the prior art. In particular, thetotal failure of the press cover as a result of even only local damageto the same because of overloading during normal operation is to beprevented or its consequences detected in good time. In particular, thestoppage times of a pressing device equipped with such a press cover areto be reduced.

The object is achieved by the features of the independent claims.Particularly preferred and advantageous embodiments of the invention arereproduced in the sub claims.

The inventors have recognized that local overloading that has alreadytaken place can be detected better on a press cover if there is asufficiently high optical contrast between the reinforcing structure andthe at least one polymer layer in which the latter is embedded. This isbecause if the overload is sufficiently high, this normally leads tolocal damage to the reinforcing structure, for example in the form oftears in the threads. Torn threads can then be detected better by thehuman eye, so that a planned change of the press cover in good time canbe carried out.

To this end, according to the invention, the materials of thereinforcing structure and of the at least one polymer layer are chosensuch that the visible light transmittances thereof differ from eachother. If the transmittance of the material of the at least one polymerlayer is chosen to be higher than that of the reinforcing structure,then the reinforcing structure can shine through the polymer layer underappropriate lighting conditions. It would also be possible to say thatthe material of the at least one polymer layer is more transparent ormore see-through (that is to say less opaque) than that of thereinforcing structure. Alternatively, the press cover can be illuminatedor shone through with light that is visible (to the human eye), forexample from an LED lamp. Local overloading can then be detectedoptically, for example with the machine at creep speed, at which thepress cover rotates more slowly than in normal operation, in which themachine produces the fibrous web. Then, an impending loss of oil fromthe press cover can be avoided in good time by means of an appropriatereplacement of the press cover.

In optics, the transmittance describes the proportion of the incidentradiation flux or light current which passes completely through atransparent component. The incident radiation flux which strikes thecomponent can be transmitted by the latter, reflected and absorbed.Accordingly, the result is the following power balance of the radiationflux striking the component in a transmittance (T), a reflectance (R)and an absorptance (A):

R+A+T=1,

which are divided among one another in accordance with the materialproperties of the component. The transmittance therefore corresponds tothat part of the incident radiation flux which is reduced by thereflectance (R) and the absorptance (A). Another designation for thetransmittance is the term total transmission T_(t). The last-named ortransmittance corresponds the ratio of the reciprocal of the incidentradiation flux T₁ to the radiation flux T₂ let through by thetransparent component, that is to say T₂/T₁. Therefore, if it ismentioned that, for example, the material of the at least one polymerlayer has a transmittance of 50%, then said layer lets through only halfof the incident radiant flux. The transmittance or the totaltransmission are defined and can be measured in accordance with ASTM D1003-00. The measurement can be carried out at any desired point on thepress cover, for example even at its axial edges, thus, for example, inthe region of the lugs by means of which such a press cover is held onthe two lateral tensioning disks. The statement of the transmittance canrelate to a factory-fresh, that is to say finished, press cover.

If it is mentioned that a polymer layer or the press cover is uncolored,then this means that it is free of colorants. Colorants can bedyes—which dissolve in the polymer layer at the molecular level or areadsorbed on the surface of the latter—or pigments, that is to sayparticles that are insoluble in the material of the polymer layer.

In the sense of the invention, a press cover is to be understood as abelt, hose or a cover which is closed, endless about its longitudinalaxis in the circumferential direction, which, as illustrated, is guidedthrough the nip (press nip) of a shoe press together with a fibrous web.To dewater the fibrous web, in normal operation the radially outermostsurface (polymer layer) of the press cover can come into contact with apress felt, by which the fibrous web to be dewatered is carrieddirectly. Depending on the embodiment of the pressing device, forexample to smooth the latter, the press cover can also come directlyinto contact with the fibrous web in normal operation. At its axialends—as viewed in the width direction (along the longitudinal axis)—itis open. Therefore, the press cover can be held at these axial ends bytwo lateral tensioning disks in order to form the shoe press roll.Instead of the guidance by the two lateral tensioning disks, the presscover can be guided over the press shoe and multiple guide rolls, as isthe case in open shoe presses. Irrespective of whether the press coveris guided by the tensioning disks or the guide rolls, the press shoe(and the guide rolls) come (temporarily) into contact with part of theradially innermost surface of the press cover. The radially outermostsurface of such a press cover, that is to say, for example, the radiallyoutermost polymer layer of the same, can be provided with grooves and/orblind drilled holes.

Longitudinal direction means that direction which extends parallel tothe longitudinal axis of the press cover. The longitudinal axissimultaneously corresponds to the axis of symmetry or rotation of thefinished press cover and the press roll. The circumferential directionof the press cover, viewed around its radial limit, extends around thelongitudinal axis. The term parallel also includes those angulardeviations of two reinforcing threads lying in different planes of +/−5°relative to each other.

The press cover or the at least one polymer layer can be produced partlyor completely from a polymer. Here, a castable, curable, preferablyelastomeric polymer such as polyurethane can be used as the polymer.Consequently, the polymer can be set as a cast elastomer.

Polymer layer means a layer which comprises such a castable, curable,preferably elastomeric polymer or is produced entirely therefrom.Preferably, the polymer layer can be a cured layer produced in one pieceby primary molding. In other words, this is molded monolithically, thatis to say produced by casting, for example. The term in one piece alsoincludes cases in which one layer has in turn been produced frommultiple layers of the same material during the casting of the polymer.However, this only to the extent that these layers are substantially nolonger visible following the curing, instead a single, preferablyuniform layer results. The same is correspondingly true of the finishedpress cover.

If a plurality of polymer layers is provided, these can be arranged oneabove another, at least in some sections, over the width of the presscover—as viewed in the radial direction. At least some sections over thewidth of the press cover means that the press cover is, for example,only single-layered at its axial ends along the longitudinal axis of thepress cover, whereas it is two-layered or multi-layered between theaxial ends. However, the polymer layers can also extend over the entirewidth of the press cover. In addition, the thickness of the presscover—and thus the thickness of the individual polymer layers—can varyin some sections along the longitudinal axis in a section through thelatter. Thus, for example, the radially outermost polymer layer in theregion of the width edges of the press cover can be smaller than in thecenter of the press cover. In other words, in the region of the widthedges, the radially outermost polymer layer can be less thick than aradially inner or radially innermost polymer layer. Preferably, exactlyone, two or three polymer layers is/are provided. These can be designedidentically with regard to their polymer or vary with regard to thehardness or stoichiometry of the pre-polymer.

A total thickness of the finished press cover in a section through thelongitudinal axis of the same measured in the radial direction can be 5to 10 mm, preferably 5 to 7, particularly preferably 5 to 6 mm.According to the invention, when a single layer is provided, the presscover can be produced from only one casting, i.e. monolithically, sothat the single layer has the thickness just mentioned.

A finished press cover in the sense of the invention is one of which theat least one polymer layer is cured and possibly finally machined, thatis to say is ready to use for the purpose mentioned at the beginning,for example in a shoe press. In an analogous way, a finished polymerlayer means a layer which is cured.

In the sense of the invention, a reinforcing thread is understood to bea flexible, textile linear structure which has a dominant extent and auniformity in its in longitudinal direction. If fibers are mentioned,then this means a single, endless fiber in the manner of a monofilament.If, on the other hand, mention is made of a fiber bundle in the sense ofthe invention, this is not monofilaments but, for its part, anindividual thread such as a twine or yarn, that is to say a bundle ofendless fibers or monofilaments. The fiber bundles themselves canentirely possibly be produced from fibers twisted with one another.

The definition that at least the longitudinal threads are produced asreinforcing threads according to the invention means that only thelongitudinal threads are designed in this way or, in addition, thelongitudinal threads and at least one further circumferential thread areproduced in this way. If, preferably, for example a laid fabric made ofcircumferential and longitudinal threads is present, then this meansthat at least the longitudinal threads are designed according to theinvention.

In the sense of the invention, the term reinforcing structure means areinforcement of the at least one layer containing the polymer orconsisting of the latter—that is to say the polymer layer. Here, thereinforcing structure can be embedded completely in the polymer layer,so that the reinforcing structure does not go beyond the limit of thepolymer layer. In other words, the polymer layer performs the role of amatrix, which surrounds the reinforcing structure and bonds it to thematrix as a result of adhesive or cohesive forces. Such a reinforcingstructure can comprise textile linear structures—e.g. yarns ortwines—and/or two-dimensional textile structures—such as, for example,woven fabrics, knitted fabrics, crocheted fabrics, braided fabrics orlaid fabrics—and be producible from an appropriate starting material,for example by winding. In other words, a single reinforcing threadaccording to the invention, viewed on its own, can be a textile linearstructure. A plurality of such reinforcing threads can be designed, forexample, as longitudinal and/or circumferential threads such that theytogether form a textile two-dimensional structure. The at least onereinforcing thread which is embedded in the at least one polymer layerthen represents the reinforcing structure of the press cover or thepolymer layer of the latter.

Starting material is understood to be that material or semifinishedproduct by means of which the reinforcing structure of the finishedpress cover according to the invention is produced, that is to say inthe present case the at least one reinforcing thread.

The reinforcing thread or the reinforcing structure can be produced froma polymer or comprise one such. Suitable polymers are polyesters,polyethylene naphthalate or polyamides such as aramides. Therefore, thematerials of the at least one polymer layer and that of the at least onereinforcing thread or the reinforcing structure embedded therein differ.

In the sense of the invention, a pressing device means, for example, ashoe press, for example for dewatering or treating, such as smoothing, afibrous web. The shoe press comprises a shoe press roll and an opposingroll, which together form or delimit a press nip. The shoe press rollfurther comprises a circulating press cover and a stationary pressingelement, the so-called press shoe. The latter is supported on aload-bearing, likewise stationary, yoke—for example via hydraulicpressing elements—and is pressed onto the circulating press cover. Thepress cover circulates relative to the stationary press shoe and yokeand, as a result, is pressed onto the opposing roll in the press nip.The press shoe and yoke are arranged radially within the press cover.The term stationary is understood to mean that the pressing element doesnot circulate relative to the shoe press roll or the opposing roll butcan move translationally—toward the opposing roll and away from thelatter, preferably in the radial direction thereof—and thereforerelative to the opposing roll. In addition to the fibrous web and presscover, one or more press felts circulating endlessly in thecircumferential direction and/or further endlessly circulating pressbelts can be guided through the press nip of the shoe press. Of course,such a shoe press can comprise more than one press nip.

In the sense of the invention, a fibrous web is a laid fabric or tangledfabric of fibers, such as wood fibers, plastic fibers, glass fibers,carbon fibers, additional materials, additives or the like. Thus, thefibrous web can be formed, for example, as a paper, board or tissue web.It can substantially comprise wood fibers, wherein small quantities ofother fibers or else additional materials and additives can be present.This is left up to those skilled in the art, depending on use.

If, preferably, a plurality of reinforcing threads as longitudinalthreads and at least one reinforcing thread as a circumferential thread,which surrounds the longitudinal threads in the circumferentialdirection, are embedded into the polymer layer as a laid fabric, thenthe advantages according to the invention are particularly wellachieved. This is because a laid fabric is capable of absorbing localoverloads particularly well.

The advantages according to the invention are achieved particularly wellif the transmittance of the material of the at least one polymer layeris between 50% and 90%—even up to 100%—and preferably between 50% and75% and, preferably, the transmittance of the material of the at leastone polymer layer is higher than the transmittance of the material ofthe reinforcing structure, which is preferably more than 1.1 to 1.5times the last-named transmittance.

The advantages according to the invention are achieved particularly wellif the press cover is built up from preferably a plurality of polymerlayers arranged one above another in the radial direction. If twopolymer layers are provided, then the radially inner one is that havingthe reinforcing structure according to the invention. This means thatthe reinforcing structure is arranged only in the radially innermostpolymer layer. If three or more polymer layers are provided, then thereinforcing structure is preferably arranged in the second lowestpolymer layer, that is to say in that which lies radially above theradially innermost polymer layer.

The invention also relates to a press roll, such as a shoe press roll,for a shoe press for dewatering a fibrous web, wherein the press rollhas at least one press cover according to the invention.

The invention also relates to a shoe press for dewatering a fibrous web,preferably a paper, board, tissue or pulp web, comprising a press rolland an opposing roll, which together form or delimit a nip, wherein thepress roll comprises a circulating press cover, wherein the press coveris formed according to the invention.

Finally, the invention relates to the use of a press cover according tothe invention for a press, such as a shoe press for dewatering a fibrousweb, preferably a paper, board, tissue or pulp web.

The invention will be explained in more detail below with reference tothe drawings without restricting the generality. In the drawings:

FIG. 1 shows a partly sectioned, schematic side according to anexemplary embodiment of the present invention;

FIGS. 2a & 2 b show embodiments of a press cover, each in a sectionviewed through its longitudinal axis;

FIG. 3 shows a highly schematic illustration of a device for producingthe press cover in a side view.

FIG. 1 illustrates a partly sectioned schematic side view of a shoepress 10 which, in the present case, comprises a press roll according tothe invention, such as a shoe press roll 12, and an opposing roll 14.The shoe press roll 12 and opposing roll 14 are arranged parallel toeach other with respect to their longitudinal axes. Together they form anip 22 or delimit such a one.

While the opposing roll 14 here comprises a cylindrically configuredroll rotating about its longitudinal axis, the shoe press roll 12 isassembled from a shoe 16, a stationary yoke 18 carrying the latter and apress cover 20. The shoe 16 and yoke 18 are arranged to be stationary inrelation to the opposing roll 14 and the press cover 20. This means thatthey do not rotate. The shoe 16 is supported by the yoke 18 and, viahydraulic press elements not illustrated, is pressed against theradially innermost surface of the press cover 20 circulating relativethereto. The press cover 20, surrounds the shoe 16 and the yoke 18 inthe circumferential direction and in the process rotates about itslongitudinal axis in the opposite direction of rotation to the opposingroll 14. Because of the concave configuration of the shoe 16 on its sidefacing the opposing roll 14, the result is a comparatively long nip 22.

The shoe press 10 is suitable in particular for dewatering fibrous webs24. During the operation of the shoe press, a fibrous web 24 is guidedthrough the press nip 22 with one or two press felts 26, 26′. In thepresent case, there are exactly two press felts 26, 26′, whichaccommodate the fibrous web 24 between them in the manner of a sandwich.During the passage through the nip 22, a pressure is exerted indirectlyon the fibrous web 24 by the press felts 26, 26′ in the nip 22. Thistakes place as a result of the fact that the radially outermost surfaceof the opposing roll 14, on the one hand, and the radially outermostsurface of the press cover 20 come directly into contact with thecorresponding press felts 26, 26′. The liquid escaping from the fibrousweb 24 is picked up temporarily by the press felt or press felts 26, 26′and any depressions (not illustrated) provided in the press coversurface. After leaving the nip 22, the liquid picked up by thedepressions of the press cover 20 is thrown off before the press cover20 enters the press nip 22 again. In addition, the water picked up bythe press felt 26, 26′ can be removed by suction elements after leavingthe press nip 22.

In a further embodiment of the invention, not illustrated in thefigures, the press felts 26, 26′ can be omitted. In such a case, thefibrous web 24 is in direct contact on one side with the press cover 20and on the other side with the opposing roll 14, which together form apress nip. Said opposing roll 14 can be designed as a heated dryingcylinder.

As illustrated in the following figures, the press cover illustrated inFIG. 1 can be designed according to the invention.

In FIGS. 2a and 2b , different embodiments of the invention areillustrated in a cross section, not to scale and partly illustrated,through the longitudinal axis 20′ of the finished press cover 20. Thedistance of the longitudinal axis 20′ to the radially innermost surfaceof the corresponding polymer layer of the press cover 20 is likewiseillustrated not to scale.

According to FIG. 2a , exactly two polymer layers are provided, namely afirst 20.1 and a second 20.2. In the present case, the first polymerlayer 20.1 is at the same time the radially outermost polymer layer ofthe press cover 20. The two polymer layers 20.1, 20.2 adjoin each otherdirectly as viewed in the radial direction, that is to say there is nointermediate layer between these two.

As illustrated, a reinforcing structure 20″ can be provided in thesecond polymer layer 20.2. In the present case, this is embeddedcompletely in the polymer layer 20.2. This is indicated by the hatchedcircles, which can be textile two-dimensional or linear structures suchas fibers. This means that the reinforcing structure 20″ does not extendbeyond the limits of the polymer layer 20.2.

Here, the reinforcing structure 20″ comprises a plurality of reinforcingthreads 21 serving as longitudinal threads 21.1. These are arranged inthe longitudinal direction of the press cover 20, extending parallel toone another at a distance over its circumference. In addition, here atleast one further reinforcing thread 21 is provided as a circumferentialthread 21.2, which preferably extends in the form of a helix in thecircumferential direction of the press cover within the same polymerlayer 20.1, 20.2, 20.3 in which the longitudinal threads 21.1 are alsoarranged. The longitudinal threads 21.1 and the circumferential thread21.2 form a laid fabric with one another, specifically in such a waythat the longitudinal threads 21.1 are arranged radially within the atleast one circumferential thread 21.2—viewed in relation to thelongitudinal axis 20′ of the press cover 20.

In the present case, the first and a second polymer layer 20.1, 20.2 areproduced from a polyurethane. This can be obtained, for example, from aprepolymer and a cross-linker. The respective prepolymer itself can beobtained by reacting an isocyanate with a polyol.

FIG. 2b shows, in a modification relative to FIG. 2a , a three-layerpress cover. This comprises a—here radially outermost—first polymerlayer 20.1, a radially innermost, third polymer layer 20.3 and a secondpolymer layer 20.2 arranged between these two in the manner of asandwich. The arrangement relates—as also in the illustration of FIG. 2a—starting from the longitudinal axis 20′ of the press cover 20 viewed inits radial direction. In the present case, a (single) reinforcingstructure 20″ is provided only in the second polymer layer 20.2. Ofcourse, this could also be different, so that, alternatively oradditionally, such a reinforcing structure 20″ could also be arranged inthe first polymer layer 20.1 and/or the third polymer layer 20.3. Thatwhich was explained in relation to the reinforcing structure 20″ in FIG.2a also applies analogously.

It has transpired that the advantages according to the invention can beimplemented particularly well if the polyurethane of the at least onepolymer layer 20.1, 20.2, 20.3 or all the polymer layers is chosen insuch a way that it is more transparent than the material of thereinforcing structure 20″. Therefore, in other words, that the materialof the reinforcing structure 20″ and that of the at least one polymerlayer 20.1, 20.2, 20.3 are chosen in such a way that their respectivetransmittance for visible light differs from one another and,preferably, the transmittance of the material of the at least onepolymer layer 20.1, 20.2 exceeds the transmittance of the material ofthe reinforcing structure 20″.

FIG. 3 shows a highly schematic side view of a device for producing apress cover 20 according to the invention. In the present case, thedevice has exactly one cylindrical mandrel 4, wherein here, for example,a starting material 20″′ is applied spirally to its radially outermostlateral surface. After it has been embedded in the polymer, the startingmaterial 20″′ forms the reinforcing structure 20″ of the finished presscover 20 according to the invention.

The illustration shows a starting stage of the production process. Inthe present case, for this purpose one end of the starting material 20″′is fixed to a polymer, which is arranged on the outer circumference ofthe mandrel 4. Apart from the schematic illustration shown, one end ofthe starting material 20″′ could also be laid or applied directly to themandrel 4 without a polymer initially being provided between thestarting material 20″′ and the mandrel 4. The starting material 20″′ canbe a textile two-dimensional structure or linear structure.

The mandrel 4 is mounted such that it can rotate about its longitudinalaxis 20′, which corresponds to the longitudinal axis of the press coverto be produced. The longitudinal axis 20′ here extends perpendicularlyinto the drawing plane. Via a line 5, a casting material, such ascastable, curable elastomeric polymer, for example polyurethane, isdischarged through a casting nozzle 6 from above onto the radiallyoutermost lateral surface of the mandrel 4 or onto the starting material20″′. Such a casting material can be chosen, for example with regard toits pot time and viscosity, in such a way that it does not drip off themandrel 4 during the casting. During this, the mandrel 4 is rotatedabout its longitudinal axis in the direction of the arrow. At the sametime as this rotation, the casting nozzle 6 is guided along on themandrel 4 along the longitudinal axis 20′ and parallel to the latter viaa suitable guide, not further illustrated in FIG. 3. At the same time asthe casting material is poured on, the starting material 20″′ is rolledonto the rotating mandrel 4 to form spirals. In the process, castingmaterial can pass through the starting material 20″′ as far as themandrel 4. In this example, after the curing step the polymer forms aradially innermost and preferably elastomeric polymer layer, whichcorresponds to the polymer layer 20.2 of the press cover from FIG. 2a ,of which only part is shown in FIG. 3.

The casting material emerging from the casting nozzle 6 in the presentcase is a mixture of a prepolymer and a cross-linker. The first isprovided from a prepolymer container, not shown, in which it is storedor stirred. The prepolymer is the reaction product of an isocyanateaccording to the invention and a polyol. In the prepolymer container, itcan be present, for example, in the form of a prepolymer made of thematerials just mentioned.

The cross-linker can be provided in a cross-linker container.

The prepolymer container and cross-linker container are assigned to thedevice for producing a press cover 20. They are connected in aflow-conducting manner to a mixing chamber (not illustrated) connectedupstream of the casting nozzle 6 in the flow direction via lines,likewise not illustrated. The prepolymer-cross-linker mixture istherefore produced upstream and outside the casting nozzle 6, that is tosay mixed in the mixing chamber. Irrespective of the production of themixture, this is then applied to the surface of the mandrel 4 to formthe at least one polymer layer of the press cover 20.

In principle, it would be conceivable that two or more casting nozzles 6could be provided. These could be connected via appropriate lines toseparate prepolymer and cross-linker containers, in order also to supplydifferent polymers to the plurality of casting nozzles 6 independentlyof one another. The casting nozzles 6 could then be arranged along thelongitudinal axis of the press cover 20 at a distance from one anotherin order to produce a plurality of polymer layers 20.1, 20.2, 20.3simultaneously in one casting by the simultaneous discharge of thepolymer out of the casting nozzle 6.

By means of such a continuous casting operation, which is also known asrotational casting, an endless, intrinsically closed cylindrical presscover 20, the inner circumference of which corresponds closely to theouter circumference of the mandrel 4, is therefore produced graduallyover the width of the mandrel 4.

In principle, it would be conceivable to wind the starting material 20″′onto more than the one mandrel 4 shown in FIG. 3. For example, twomandrels could be provided, which could be arranged parallel to oneanother at a distance with regard to their longitudinal axes.Alternatively, it would also be conceivable to apply the polymer also tothe radially inner lateral surface of the mandrel 4, for example in themanner of centrifuging. Irrespective of the embodiment mentioned, thefinished press cover 20 is finally taken off the at least one mandrel 4.

As illustrated in FIG. 3, the press cover 20 is designed according tothe invention.

Although this is not illustrated in the figures, the reinforcingstructure 20″ of the at least one polymer layer 20.1, 20.2 could also bebuilt up from a plurality of starting materials 20″′ laid over oneanother in the radial direction, each extending in the longitudinalaxial direction and in the circumferential direction of the press cover20.

1-16. (canceled)
 17. A press cover, comprising: at least one polymerlayer; and a reinforcing structure embedded in said at least one polymerlayer, said reinforcing structure having at least one reinforcingthread, wherein a material of said reinforcing structure and that ofsaid at least one polymer layer are chosen in such a way that visiblelight transmittances thereof differ from one another.
 18. The presscover according to claim 17, wherein a transmittance of said material ofsaid at least one polymer layer is higher than a transmittance of saidreinforcing structure.
 19. The press cover according to claim 17,wherein a transmittance of said material of said at least one polymerlayer is between 50% and 90%.
 20. The press cover according to claim 17,wherein: said at least one polymer layer is produced from polyurethaneor contains said polyurethane; and said at least one reinforcing threadis produced from a polymer or contains said polymer, said polymer isselected from the group consisting of a polyester, polyethylenenaphthalate and polyamide.
 21. The press cover according to claim 17,wherein said at least one polymer layer is uncolored.
 22. The presscover according to claim 17, wherein said at least one reinforcingthread is provided as a longitudinal thread, which extends in alongitudinal direction of the press cover.
 23. The press cover accordingto claim 22, wherein said at least one reinforcing thread is one of aplurality of reinforcing threads provided as longitudinal threads which,extend in the longitudinal direction of the press cover, and aredisposed parallel to one another at a distance over a circumference ofthe press cover.
 24. The press cover according to claim 23, furthercomprising at least one further reinforcing thread as a circumferentialthread.
 25. The press cover according to claim 24, wherein saidreinforcing threads configured as said longitudinal threads and said atleast one further reinforcing thread configured as said circumferentialthread form a laid fabric with one another.
 26. The press coveraccording to claim 25, wherein as viewed in a radial direction of thepress cover, said longitudinal threads are spaced apart in the radialdirection relative to said circumferential thread at crossing points.27. The press cover according to claim 17, wherein said at least onepolymer layer is one of a plurality of polymer layers, said at least onepolymer layer, viewed in relation to a longitudinal axis of the presscover, is a radially inner or innermost polymer layer and, a furtherpolymer layer of said polymer layers is a radially outermost polymerlayer of said polymer layers, viewed in relation to the longitudinalaxis of the press cover.
 28. The press cover according to claim 27,wherein said plurality of polymer layers is exactly two polymer layers,and said radially inner polymer layer is at a same time the radiallyinnermost polymer layer of the press cover.
 29. The press coveraccording to claim 18, wherein the transmittance of said material ofsaid at least one polymer layer is 1.1. to 1.5 times higher than thetransmittance of said reinforcing structure.
 30. The press coveraccording to claim 19, wherein the transmittance of said material ofsaid at least one polymer layer is between 50% and 75%.
 31. The presscover according to claim 24, wherein said circumferential thread extendsin a form of a helix in a circumferential direction of the press coverwithin the at least one polymer layer.
 32. The press cover according toclaim 25, wherein said reinforcing threads configured as saidlongitudinal threads and said at least one further reinforcing threadconfigured as said circumferential thread form said laid fabric in sucha way that said longitudinal threads are disposed radially within saidcircumferential thread, viewed in relation to a longitudinal axis of thepress cover.
 33. A press roll for a shoe press for treating a fibrousweb, the press roll comprising: at least one press cover according toclaim
 17. 34. A shoe press for treating a fibrous web, the shoe presscomprising: a press roll and an opposing roll which together form ordelimit a nip, wherein said press roll containing a circulating presscover, said circulating press cover being formed according to claim 17.35. A method of using a press cover, which comprises the steps of:providing the press cover according to claim 17 for a press for treatinga fibrous web.
 36. A method of building a structure, which comprises thesteps of: disposing at least one reinforcing thread, functioning as areinforcing structure, in a press cover, the press cover having at leastone polymer layer in which the reinforcing structure is embedded,wherein a material of the reinforcing structure and that of the at leastone polymer layer are chosen in such a way that visible lighttransmittances thereof differ from one another.